Some of the ones i have made and used on a regular basis.
First up, woods vice, all it is is a stump from a small sapling cut high at a nice working height, a piece of cord is secured at the bottom of the split to stop it running, the work piece is pushed into the split (you can hollow it out a bit too to make life easier, another piece of cord is then put over the top and twisted with a handle (aka a stick) to close the split and thus gripping the work piece, the handle is then stopped from spinning back by laying it alongside the stump and securing with a loop of cord.
A picture makes things easier to understand, i can only find this small one at present, if i can find the original on my pooter i'll upload it.
Wedges are your friend for securing things though, simple and effective as seen in these that i used on my bowl horse
Another variation of this is a the carving stump i made yonks ago, all it was was a large round of Oak with lots of 1" holes augered in it to give me all sorts of options for securing work with pegs ie: bits of stick, i had a couple of large pegs at the rear and then secured the front and sides with pegs using the adjacent holes, then wedging where needed, pictures show what i mean.
Sawhorse wise, my favourite made horse is this one, being tripods they are always stable on uneven ground, the thing i like about this style is thet are simple and quick to make, need minimal tools and materials and you can move them closer or further apart depending on the length of timber you are cutting
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Of course there is also a lot to be said for the simple sawhorse/bench/fallen tree and a well placed knee or foot to secure the workpiece
The simplest clamp i have used though is just a long loop of strong cordage placed over a raised fallen tree, put the work piece under the loop on the top of the tree, then put your foot in the bottom of the loop and press down, or like a pole lathe treadle, put a length of wood in the bottom of the loop and stand on it, the harder you press, the tighter it grips.
Then of course you can get into more elaborate things like shave horses and the like.
BTW, i wouldn't suggest using any of these in your kitchen unless you have a very understanding better half.